Palaeohydraulic modelling is presented for Athabasca Vallis, the youngest known catastrophic flood channel on Mars. This modelling incorporates three significant advantages over previous modelling of Martian channels: a step-backwater hydraulic model; more accurate topography; and improved flood height indicators. The maximum modelled palaeodischarge is between 1 × 106 and 2 × 106m3s−1 depending on the friction coefficient selected. An anomalously high palaeostage indicator suggests a region of ponded backwater in the channel in which streamlined forms were created through deposition, with the additional possibility of post-flood subsidence/lowering of the channel slope due to magma extrusion. Résumé On présente une modélisation paléo-hydraulique d'Athabasca Vallis, le plus jeune chenal de crue catastrophique connu sur Mars. Cette modélisation présente trois avantages majeurs par rapport aux modélisations précédentes des chenaux de Mars: un modèle hydraulique à casiers; une topographie plus précise; et des indicateurs de niveaux d'eau plus proches de la réalité. La valeur modélisée maximale de paléo-débit se situe entre 1 × 106 et 2 × 106 m3 s−1 selon le coefficient de frottement choisi. Un indicateur de paléo-niveau anormalement haut suggère qu'il y avait des eaux stagnantes dans le chenal au sein duquel des formes d'écoulement se sont créées à travers les dépôts, avec en plus l'éventualité d'une subsidence post-crue et diminution de la pente du chenal, liées à une extrusion magmatique.

Central pit craters (CPCs) are complex craters that exhibit depressions within their floors or central uplifts. Some proposed models for central pit formation can be grouped by similar mechanism: (A) explosive volatile release, (B) central peak collapse, and (C) drainage of subsurface melted volatiles. We test these groups of mechanisms for central pit formation on Mars based on morphometries expected from each mechanism. Volatiles in the target (A and C) would be expected to result in a correlation with layered ejecta, also formed from volatiles, for CPCs over non‐CPCs. Central peak collapse (B) should result in larger diameters for pit rims than central peaks due to outward flow. Explosive volatile release (A) should yield smaller volumes for pit rims than pits due to mass loss. Data were collected on Context Camera images and digital elevation models for random samples of CPCs to assess the presence of these expected morphologies. Results of a Fisher Exact test showed no preference in layered ejecta with CPCs over non‐CPCs, inconsistent with volatiles in the target. Results of an independent t‐test showed central pit rim diameters are larger than central peak diameters, supporting some component of central peak collapse, although measurement uncertainty makes this interpretation tentative. Central pit rim volumes were not found to be statistically smaller than their pits, weighing against formation by explosive volatile release. Thus, our findings do not support any single group of formation mechanism tested here, although they allow for formation by some combination of these (or other) processes.

Grain sizes of Martian sand dunes are critical sedimentological data on sand provenance and transport pathways. Thermal inertia values are used to characterize the grain sizes of dune sand. Most early characterizations involved single dune fields. Recent work based on global data sets has provided more wide‐spread dune sand locations, though these data sets include the non‐sandy interdune areas. To provide a more accurate grain size characterization, we leverage a global thermal inertia data set, a global dune database and a global imaging mosaic to develop a freeware‐based methodology for deriving grain sizes. This methodology involves delineation of sand‐only areas within dune fields and collection of thermal inertia values from those areas. We consider a unimodal histogram of values with a mode <∼350 thermal inertia units (J m⁻² K⁻¹ s^−1/2) to imply an effective exclusion of non‐sand surfaces. Application of this methodology to dune fields for which thermal inertia values have been previous derived shows our results fall within the envelope of those values. We apply our methodology to tropical dune fields on Mars for which Dust Cover Index data imply dust‐free surfaces. Conversion of these thermal inertia values to sand grain sizes yields a range of sand classifications of fine sand to granules. Comparison of sand size classifications with geographic location shows grain size ranges that are distinctive by location, consistent with local sourcing. This work points toward geographically diverse sand formation mechanisms yielding diverse grain sizes, while providing a freeware‐based and thus widely accessible method for expanding the derivation of these critical data.